Same symptoms here after upgrade:
1) Widgets as previously installed all present & working - except Contacts
2) Clicking the plus button shows an empty screen of available/installable widgets
3) Trashing plist files & double clicking from library folder does install widget but doesn't fix the empty screen
I'm thinking that there is buried in the OS somewhere a setting that tells Dashboard where to "see" available widgets and for some reason it ain't "seeing".
PS. I got Contacts widget back by double clicking it in Library folder and then agreeing to the security warning about Dashboard accessing my contacts.
PPS. Have just checked a "vanilla" admin account on same machine and widget screen looks OK there - so definitely a setting somewhere within the user space is wonky.
Message was edited by: Kiwi Graham